The present invention relates to atomic absorption spectrophotometers and, more particularly, to an atomic absorption spectrophotometer of type in which a plurality of light beams from respective light sources are efficiently introduced into a single sample atomization compartment, for measuring and analyzing a plurality of elements simultaneously.
An atomic absorption spectrophotometer is known, in which a plurality of light beams emitted from respective light sources are brought to a single light beam by the use of semitransmissive mirrors, and the single light beam is introduced into an atomization compartment of a sample (Elik Lundberg, Gills Johansson, Anal. Chem., 48, 1922-1925 (1976)).
An atomic absorption spectrophotometer is also known, in which a single light source emitting a continuous spectrum is used to carry out a simultaneous multielement measurement (J. M. Harnly, T.C.O' Haver, B. Golden, W. R. Wolf, Anal. Chem., 51, 2007-2014 (1979)).
On the other hand, an atomic absorption spectrophotometer is known, which is of multi-channel type in which a chemical combustion flame is employed as a method of atomizing a sample, and three light beams from respective hollow-cathode lamps are introduced into the chemical combustion flame (Kazuo Hotta and Takahiko Hasegawa, "Atomic Absorption Analysis", Kodan-Sha, Scientific, pp. 121-122 (1972)).
Wavelengths of atomic absorption lines and wavelengths of proximity lines of other elements are disclosed, for example, in R. J. Lovett, D. L. Welch, M. L. Parson, Applied Spectroscopy, 29, 470-477 (1975). The typical wavelengths are indicated in the below table.
TABLE I ______________________________________ Atomic Absorption Line Wavelengths And Proximity line Wavelengths Of Other Elements Unit: nm Atomic Absorption Proximity Line Differential Line Wavelengths Wavelengths Wavelengths ______________________________________ Al 308.215 V 308.211 0.004 Sb 217.023 Pb 216.999 0.024 Sb 231.147 Ni 231.097 0.050 Cd 228.802 As 228.812 0.010 Ca 422.673 Ge 422.657 0.016 Co 252.136 In 252.137 0.001 Cu 324.754 Eu 324.753 0.001 Ga 403.298 Mn 403.298 0.000 Fe 271.903 Pt 271.904 0.001 Mn 403.307 Ga 403.298 0.009 Hg 253.652 Co 253.649 0.003 Si 250.690 V 250.690 0.000 Zn 213.856 Fe 213.859 0.003 B 247.773 Ge 249.796 0.023 Bi 202.121 Au 202.138 0.017 Co 227.449 Re 227.462 0.013 Cu 216.509 Cu 216.517 0.008 Ga 294.418 W 294.440 0.022 Au 242.795 Sr 242.810 0.015 In 303.936 Ge 303.906 0.030 Fe 248.327 Sn 248.339 0.012 Pb 261.365 W 261.382 0.017 Mo 379.825 Nb 379.812 0.013 Os 247.684 Ni 247.687 0.003 Pd 363.470 Ru 363.493 0.023 Pt 227.438 Co 227.449 0.011 Pn 350.252 Co 350.262 0.010 Sc 298.075 Hf 298.081 0.006 Si 252.411 Fe 252.429 0.018 Ag 328.068 Rh 328.060 0.008 Tl 291.832 Hf 291.858 0.026 Ti 264.664 Pt 264.689 0.025 ______________________________________
FIG. 5 shows an atomic absorption spectrophotometer of simultaneous multi-element analysis type in which a plurality of light beams emitted respectively from a plurality of light sources are brought to a single light beam by the use of semitransmissive mirrors, and the single light beam is introduced into an atomization compartment for a sample.
The atomic absorption spectrophotometer shown in FIG. 5 comprises a power source 1A, and a plurality of hollow-cathode discharge tubes 2A, 3A, 4A and 5A which serve respectively as light sources and which are turned on by electric power from the power source 1A. The hollow-cathode discharge tube of the light source 2A contains elementary zinc (Zn) and, at discharge operation, emits a resonance absorption line of zinc, that is, emission lines having the wavelength of 213.856 nm (nanometer). Likewise, the hollow-cathode discharge tube of the light source 3A contains cadmium (Cd) and emits emission lines having the wavelength of 228.802 nm; the hollow-cathode discharge tube of the light source 4A contains lead (Pb) and emits emission lines having the wavelength of 283.3 nm; and the hollow-cathode discharge tube of the light source 5A contains arsenic (As) and emits emission lines having the wavelength of 193.7 nm. The reference numeral 6A denotes a reflecting mirror, and the reference numerals 7A, 8A and 9A designates respectively semitransmissive reflecting mirrors. The four light beams are composed by these reflecting mirrors into a single light beam 10A. The light beam 10A is incident upon a cylindrical heating furnace 11A and passes through the same. Subsequently, the light beam 10A enters an incident slit 14A of a multi-wavelength spectrophotometer 13A.
Liquid droplets of a sample 12A being analyzed are introduced into the heating furnace 11A. The sample 12A is dried, ashed and finally raised in temperature to a maximum value by electric power from a power source 16A which is controlled on the basis of a temperature-raising program incorporated in a temperature control device 15A. Thus, solutes dissolved in the sample 12A being analyzed are decomposed under high temperature, thereby obtaining atomic vapor.
The light beam 10A entering through the incident slit 14A is dispersed by a diffraction grating 17A in dependence upon the wavelengths. The resonance absorption line of zinc is incident upon a photoelectric transducer 25A, is amplified in signal by an amplifier 19A, and is inputted into a signal processing device 23A. Likewise, the resonance absorption lines of the respective cadmium, lead and arsenic are incident upon their respective photoelectric transducers 26A, 27A and 28A, are amplified by respective amplifiers 20A, 21A and 22A, and are inputted into the signal processing device 23A. The signal processing device 23A reads out the electric signals representative of the emission lines of the respective four wavelengths contained in the light beam 10A when the solute elements of the sample 12A being analyzed are atomized at high temperature within the heating furnace 11A. On the basis of the electric signals, the signal processing device 23A carries out calculation to obtain quantities of the respective four elements. The calculation results are displayed on a display unit 24A.
That is, if zinc, lead, cadmium and arsenic are contained in the atomic vapor, the resonance absorption lines of these respective elements, for example, the emission lines having the wavelength of 213.856 nm in case of zinc are absorbed within the light beam 10A. The intensities of the respective spectra at these wavelengths are measured respectively by the four photoelectric transducers 25A through 28A. Thus, the four kinds of elements contained in the sample 12A being analyzed in extremely small quantities can be measured simultaneously. If the hollow-cathode discharge tubes 2A, 3A, 4A and 5A are replaced by ones for other elements, and if the multi-wavelength spectrophotometer 13A is re-set to wavelengths of resonance absorption lines of the atoms of these respective other elements, it is made possible to simultaneously measure the four kinds of other elements.
The conventional apparatus illustrated in FIG. 5 has the following two serious defects.
The first defect is that when two resonance absorption lines are close to each other, it is difficult to arrange the photoelectric transducers 25A through 28A directly behind respective exit slits 35A, 36A, 37A and 38A.
The dispersive power of the spectrophotometer employing a usual echelette diffraction grating is approximately 0.5 nm/mm, even in the apparatus high in resolving power. Accordingly, in the above table I, the differential wavelength of 0.004 nm between 308.215 nm of Al and 308.211 nm of V, for example, corresponds to a distance of 0.008 mm at the position of the exit slits of the spectrophotometer. On the other hand, as a photoelectric transducing element, there is known a photomultiplier tube which is employed in the wavelength region of the table I, that is, in a region of ultraviolet and which is highest in sensitivity and capable of maintaining the S/N ratio of signals high. The photomultiplier tube is approximately 5 mm in size even for the smallest one. Thus, it is extremely difficult to arrange the photomultiplier tubes just behind the exit slits through which the two resonance absorption lines referred to above pass, respectively. There are many combinations in wavelength each having such relationship, as indicated in the table I. Accordingly, it has been difficult in practice, to use a single spectrophotometer simultaneously to measure elements in any combination with high absorption sensitivity.
As a diffraction grating high in dispersive power, there is also known a spectrophotometer which employs a diffraction grating of echelle type, for example. Since, however, the spectrophotometer utilizes a high order diffraction beam, that is, a beam high in the order, the light intensity capable of being taken out through an exit slit is extremely low as compared with the apparatus employing the echelette diffraction grating. Accordingly, the S/N ratio of signals is lowered. For this reason, there are few instances where the spectrophotometer employing the diffraction grating of echelle type has been put into practical use.
The second defect of the prior art illustrated in FIG. 5 is that, since the light intensity of the light beam from the light source is reduced, the S/N ratio of the optical signals is lowered.
Because the light beam from the hollow-cathode discharge tube 2A passes through the semitransmissive mirrors 7A and 8A, the light intensity of the light beam cannot but be reduced to one-fourth at the most. Since, in practice, there are also loss of absorption and loss of reflection of the light beam due to the semitransmissive mirrors 7A and 8A, a greater loss of the light intensity must be taken into account.
A composing method shown in FIG. 6 is conventionally known, which improves the defect of the above-mentioned prior art at the time the four light beams from the respective four light sources 2A through 5A are composed into the single light beam 10A. Light beams different in wavelength from each other, emitted from respective hollow-cathode discharge tubes 2B, 3B, 4B and 5B having their respective hollow cathodes different in element from each other are incident upon a diffraction grating 9B and are composed into a single light beam 10B.
As is well known, supposing that angles formed between a line normal to the diffraction grating 9B and respective light beams are .theta..sub.2, .theta..sub.3, .theta..sub.4 and .theta..sub.5, respectively, and that an angle formed between the normal line and the light beam 10B is .phi., the following relationship exists: EQU .lambda..sub.i =d.multidot.(sin .theta..sub.i +sin .phi.) (1)
where
d is the spacing between each pair of adjacent grooves in the diffraction grating (mm); PA0 .lambda. is wavelengths of the respective light beams; and PA0 i is one of 2, 3, 4 and 5.
Thus, if the positions of the respective light beams, that is, the angles .theta..sub.i formed between the line normal to the diffraction grating 9B and the respective incident light beams are previously determined in accordance with the wavelengths .lambda..sub.i of the respective resonance absorption lines emitted, it is possible to compose the light beams incident upon the diffraction grating from the four different directions, into the single light beam 10B.
According to the method as described above, it is possible to eliminate the second defect of the prior art illustrated in FIG. 5, concerning reduction in the light intensity of the light beam from the light source. Since, however, the relationship of the equation (1) exists between the light beams incident upon the diffraction grating 9B and the light beam diffracted thereby, there often occurs a case where the hollow-cathode discharge tubes 2B through 5B cannot be arranged in close relation to each other on the side of the light sources, like the first defect of the prior art illustrated in FIG. 5. Further, the width of the hollow-cathode discharge tube in the direction perpendicular to the direction in which the light beam is taken out is a common size, e.g. approximately 38 mm (1.5 inches) which is large. Accordingly, it is more difficult to put the arrangement illustrated in FIG. 6 into practical use, as compared with the difficulty in the arrangement of the photomultiplier tubes at the respective exit slits of the spectrophotometer as discussed with reference to the prior art illustrated in FIG. 5.
As an example in solving the above-mentioned difficulty on the side of the light sources, there is known the prior art shown in FIG. 7, which employs a light source emitting a light beam 10C including a continuous spectrum, for example, a xenon lamp 2C. Other construction of the prior art illustrated in FIG. 7 is the same as that of the prior art shown in FIGS. 5 and 6.
The xenon lamp 2C emits the continuous spectrum which is distributed chiefly from the region of ultraviolet to the region of visible wavelengths and which contains almost all of the resonance absorption wavelengths of the respective elements indicated in the previous table I. Accordingly, the use of the xenon lamp 2C makes it possible to improve the second defect of reduction in the light intensity of the light beam from the light source, described previously with reference to the prior art illustrated in FIG. 5. However, the use of the xenon lamp 2C is entirely ineffective with respect to the first defect, that is, the difficulty in arrangement of the photoelectric transducers, described above with reference to the prior art shown in FIG. 5.
There also exists the following defect. That is, almost all of the atomic absorption spectrophotometers conventionally put into practical use in the world employ the hollow-cathode discharge tubes as light sources. This is because of the following circumstances. As is well known, the wavelength widths of the atomic resonance absorption lines are slightly different based on the type of atoms, but are in a range of from 190 nm of the region of ultraviolet to 860 nm of the region of near-infrared wavelength. On the other hand, the dispersive power of the spectrophotometer employing the echelette diffraction grating is of the order of 0.5 nm/mm even for the spectrophotometer having high resolving power, as described previously. Accordingly, in order to take out, through the exit slit, the spectrum of the order of atomic spectrum width having 0.004 nm, the exit slit having the width of about 0.008 mm is required. In addition, if the wavelength of the spectrophotometer varies by 0.0004 nm (which corresponds to the distance of 0.0008 mm at the position of the exit slit) due, for example, to fluctuation in temperature of the installation environment, the light intensity of the light beam coming out through the exit slit is reduced by approximately 10%.
It is difficult to realize such extremely small values in the usual spectrophotometer employing the echelette diffraction grating, in the form capable of being put into practical use.
In view of such difficulty, the hollow-cathode discharge tubes, in which materials of elements being analyzed are used as the cathodes, have been utilized as light sources which emit line spectra substantially equal in spectrum width to the wavelengths of the resonance absorption lines of the respective elements being analyzed.
Finally, the prior art illustrated in FIG. 7, which employs the xenon lamp, involves unreasonable demands for the revolving power and the wavelength stability with respect to the spectrophotometer, and is lacking in practicality.
FIG. 8 shows the prior art in which a chemical combustion flame is used in substitution for the heating furnace in the above-described three prior art. In the prior art illustrated in FIG. 8, atomic absorption lines different from each other are emitted respectively from hollow-cathode discharge tubes 2D, 3D and 4D into a central portion of a chemical combustion flame 30D. An aqueous solution sample 12D being analyzed is atomized by an atomizer 31D and is introduced into the chemical combustion flame 30D. The sample 12D is evaporated and vaporized, and is dissociated into the form of atoms. If the atoms in a group contain ones which are in conformity with the elements of materials forming the respective hollow cathodes of the above-mentioned respective hollow-cathode discharge tubes, the light beams are absorbed due to the atomic absorption phenomenon. The atomic spectrum light beams emitted respectively from the hollow-cathode discharge tubes are transmitted through the chemical combustion flame 30D and are incident upon respective monochromators 32D, 33D and 34D where the wavelength components of the atomic absorption lines are selected respectively. The subsequent processing is similar to that of the prior art described previously. Thus, it is possible to simultaneously measure the three elements different from each other.
The atomization compartment for the sample, through which the three line beams are transmitted, must be provided with the following two conditions.
It is necessary that the three light beams transmitted through the chemical combustion flame 30D do not change in their respective advancing directions with the lapse of time. If the advancing directions of the respective light beams change with the lapse of time, the light intensities incident upon the respective monochromators vary, making it difficult to distinguish the variation in light intensity due to the atomic absorption from the variation in light intensity due to the change in advancing direction of the light beams. Accordingly, the chemical combustion flame must continue to be burned in a stable manner with the lapse of time.
As one other condition, it is necessary that the temperature gradient at the interior of the chemical combustion flame and at the environment thereof is symmetrical about the advancing direction of each incident light beam to prevent occurrence of the phenomenon of refraction of the light beam due to the density gradient of air formed by the temperature gradient, and that even if such phenomenon of refraction of the light beam occurs, the phenomenon of refraction does not vary with the lapse of time.
In order to form the chemical combustion flame satisfying the above-described two conditions, it has in general been utilized that a pair of nozzle bores for two kinds of gases including combustion gas and oxygen or the combustion gas and stabilizing gas containing oxygen are arranged in concentric relation to each other, to form a flame having a concentric temperature distribution. The light beam advancing toward the center of the concentric of the chemical combustion flame is not substantially refracted to the right or to the left when the light beam is transmitted through the flame. In addition, since the flame is elongated vertically, the temperature gradient in the vertical direction is low. Thus, refraction of the light beam in the vertical direction is also slight.
It is difficult to replace such chemical combustion flame by the graphite-tube electric furnaces (heating furnaces 11A, 11C shown in FIGS. 5 through 7) which have recently been utilized most numerously.
In the atomization compartment of the electric furnace system, a graphite tube loaded with a sample is heated from the room temperature to approximately 3,000 degrees C. for a short period of time of about 1 to 3 seconds, to thereby form atomic vapor high in density for the short period of time. Accordingly, it is difficult for the electric furnace system to select wavelength components of respective atomic absorption lines corresponding respectively to a plurality of elements being analyzed. Further, the electric furnace system is restricted by the transmitting direction of the light beam.
In conclusion, all the prior art discussed above is significant from the scientific point of view, but is defective when consideration is given to the practicality, that is, operability, facility in change of elements being analyzed, measuring performance and the like. For these reasons, in spite of the fact that a demand for putting the simultaneous multi-element analysis into practical use is strong, such analysis has not been put into practical use up to now. Accordingly, up to now elements have still been analyzed sequentially one by one.